Methods of preparing heat resistant, creep-resistant aluminum alloy

ABSTRACT

A heat-resistant, creep-resistant aluminum alloy containing from 10 to 30 mass % of silicon, from 3 to 10 mass % of at least either iron or nickel in total, from 1 to 6 mass % of at least one rare earth element in total, and from 1 to 3 mass % of zirconium, preferably excluding titanium, magnesium and copper, with the rest substantially consisting of aluminum, is prepared by a method including providing a rapidly cooled aluminum alloy powder, molding the powder into a pressurized powder compact, and performing hot plastic working on the compact to form a product shape such as a billet. The compact is exposed to a temperature of at least 450° C. for at least 10 seconds and not more than 30 minutes before forming the product shape by the hot plastic working.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Divisional of U.S. application Ser.10/296,142, filed Nov. 20, 2002, which is the U.S. National Phase ofPCT/JP02/02731.

TECHNICAL FIELD

[0002] The present invention relates to methods of preparing aheat-resistant, creep-resistant aluminum alloy and a billet thereof, andmore particularly, it relates to methods of preparing a heat-resistant,creep-resistant aluminum alloy suitable to be used for a componentemployable at a temperature of at least 300° C. and required to havecreep resistance and a billet thereof.

BACKGROUND ART

[0003] Japanese Patent Laying-Open No. 11-293374 discloses an aluminum(Al) powder alloy having heat resistance and wear resistance. Thisgazette shows an aluminum alloy containing at least one of silicon (Si),titanium (Ti), iron (Fe) and nickel (Ni) and magnesium (Mg) as essentialadditional elements, with the mean crystal grain size of silicon and themean grain sizes of other intermetallic compound phases not more thanprescribed values.

[0004] Japanese Patent Laying-Open No. 8-232034 discloses an aluminumpowder alloy having heat resistance and wear resistance with excellentdeformability at a high temperature. This gazette mainly shows analuminum alloy containing silicon, manganese (Mn), iron, copper (Cu) andmagnesium. The gazette also shows a method of preparing an aluminumalloy by preforming rapidly solidified powder obtained by airatomization by powder pressurization molding and thereafter performingextrusion and hot swaging.

[0005] However, it has been proved that each of the aluminum alloysshown in the aforementioned two gazettes insufficiently satisfiesperformance for serving as a member required to have creep resistance,although the same is excellent in heat resistance and wear resistance.

DISCLOSURE OF THE INVENTION

[0006] An object of the present invention is to provide aheat-resistant, creep-resistant aluminum alloy excellent in heatresistance as well as in creep resistance and a billet thereof as wellas methods of preparing the same.

[0007] The inventors have made deep study under the aforementionedobject, to find out the composition and the structure of an aluminumalloy having both of sufficient heat resistance and sufficient creepresistance.

[0008] The heat-resistant, creep-resistant aluminum alloy according tothe present invention contains at least 10 mass % and not more than 30mass % of silicon, at least 3 mass % and not more than 10 mass % of atleast either iron or nickel in total, at least 1 mass % and not morethan 6 mass % of at least one rare earth element in total and at least 1mass % and not more than 3 mass % of zirconium (Zr) with the restsubstantially consisting of aluminum, while the mean crystal grain sizeof silicon is not more than 2 μm, the mean grain size of compounds otherthan silicon is not more than 1 μm, and the mean crystal grain size ofan aluminum matrix is at least 0.2 μm and not more than 2 μm.

[0009] The heat-resistant, creep-resistant aluminum alloy according tothe present invention consists of the aluminum alloy to which silicon,iron and/or nickel, a rare earth element and zirconium are added, andcontains none of titanium, magnesium and copper dissimilarly to theconventional aluminum alloys. The aluminum alloy containing neithermagnesium nor copper can be sufficiently increased in creep resistance.While titanium hinders refinement of crystal grains when addedsimultaneously with zirconium, the aluminum alloy according to thepresent invention containing no titanium is not hindered from refinementof crystal grains.

[0010] Thus, an aluminum alloy having microcrystal grains with excellentheat resistance and creep resistance can be obtained.

[0011] The content of silicon is set to at least 10 mass % and not morethan 30 mass % since silicon crystallizes out in the alloy as siliconcrystals to contribute to improvement of wear resistance, while the wearresistance is insufficiently improved if the silicon content is lessthan 10 mass % and the material is embrittled if the silicon contentexceeds 30 mass %.

[0012] The content of at least either iron or nickel is set to at least3 mass % and not more than 10 mass % in total on the basis of thefollowing reason: Iron crystallizes a fine intermetallic compound ofaluminum iron in the aluminum matrix to improve heat resistance of thematrix. When the aluminum alloy singly contains iron without nickel, noeffect of improving heat resistance is attained if the iron content isless than 3 mass % while a large acicular intermetallic compoundcrystallizes out to embrittle the material if the iron content exceeds10 mass %.

[0013] While iron may be singly added to the aluminum alloy, theintermetallic compound of aluminum and iron is converted to a ternaryintermetallic compound of aluminum, iron and nickel to be more refinedwhen iron is compositely added along with nickel. The effect ofimproving heat resistance is reduced if the content of iron and/ornickel is less than 3 mass % in total, while the aluminum alloy isembattled if the content of iron and/or nickel exceeds 10 mass % intotal.

[0014] The content of at least one rare earth element is set to at least1 mass % and not more than 6 mass % in total since the rare earthelement has a function of improving tensile strength in the temperaturerange from the room temperature to a high temperature by reducing thesize of an intermetallic compound of aluminum and a transition metal andrefining silicon crystals. The aforementioned effect is small if thecontent of the rare earth element is less than 1 mass %, while theaforementioned effect is saturated if the content exceeds 6 mass %.

[0015] The content of zirconium is set to at least 1 mass % and not morethan 3 mass % since it is effective to add zirconium improving heatresistance simultaneously with the aforementioned rare earth elementwhile the aforementioned effect is small if the content of zirconium isless than 1 mass % and the aforementioned effect is saturated if thecontent exceeds 3 mass %.

[0016] The mean crystal grain size of silicon is set to not more than 2μm since voids result in high strain rate superplastic deformation ifthe mean crystal grain size of silicon exceeds 2 μm.

[0017] The mean grain size of the compounds other than silicon is set tonot more than 1 μm since high strain rate superplastic deformation ishard to attain if the mean grain size exceeds 1 μm.

[0018] The mean crystal grain size of the aluminum matrix is set to atleast 0.2 μm and not more than 2 μm since grain boundary sliding iscaused between crystal grains to develop superplasticity when stress isapplied at a temperature of at least 450° C. in this grain size range.If the mean crystal grain size of the aluminum matrix is less than 0.2μm, the strain rate developing superplasticity exceeds 10²/sec., torequire a working method such as explosive forming extremely inferior ineconomy. If the mean crystal grain size of the aluminum matrix exceeds 2μm, no superplasticity is developed or the strain rate is reduced below10⁻²/sec. following development of superplasticity, to require a longtime for hot working.

[0019] The aforementioned heat-resistant, creep-resistant aluminum alloypreferably contains at least 0.5 mass % and not more than 5 mass % of atleast one element selected from a group consisting of cobalt (Co),chromium (Cr), manganese, molybdenum (Mo), tungsten (W) and vanadium (V)in total.

[0020] These elements, not damaging the heat resistance and the creepresistance of the aluminum alloy according to the present invention, canbe added at need.

[0021] A billet of a heat-resistant, creep-resistant aluminum alloyaccording to the present invention contains at least 10 mass % and notmore than 30 mass % of silicon, at least 3 mass % and not more than 10mass % of at least either iron or nickel in total, at least 1 mass % andnot more than 6 mass % of at least one rare earth element in total andat least 1 mass % and not more than 3 mass % of zirconium whilecontaining none of titanium, magnesium and copper, with the restsubstantially containing aluminum, and has a substantially cylindricalshape.

[0022] According to the inventive billet of a heat-resistant,creep-resistant aluminum alloy, an aluminum alloy having microcrystalgrains with excellent heat resistance and creep resistance can beobtained.

[0023] In the aforementioned billet of a heat-resistant, creep-resistantaluminum alloy, elongation at 300° C. is preferably at least 1% and notmore than 7%.

[0024] Such a billet having relatively small extension can be obtainedby powder forging.

[0025] In the aforementioned billet of a heat-resistant, creep-resistantaluminum alloy, elongation at 300° C. is preferably at least 7% and notmore than 15%.

[0026] Such a billet having relatively large extension can be obtainedby powder forging.

[0027] A method of preparing a heat-resistant, creep-resistant aluminumalloy according the present invention is a method of preparing aheat-resistant, creep-resistant aluminum alloy containing at least 10mass % and not more than 30 mass % of silicon, at least 3 mass % and notmore than 10 mass % of at least either iron or nickel in total, at least1 mass % and not more than 6 mass % of at least one rare earth elementin total and at least 1 mass % and not more than 3 mass % of zirconiumwith the rest substantially consisting of aluminum, comprising a step ofmolding rapidly cooled alloy powder consisting of an aluminum alloy intoa pressurized powder compact and thereafter working the pressurizedpowder compact into a product shape by hot plastic working, while thetime exposing the pressurized powder compact not yet worked into theproduct shape to a temperature of at least 450° C. is at least 15seconds and within 30 minutes.

[0028] According to the inventive method of preparing a heat-resistant,creep-resistant aluminum alloy, the composition of the aluminum alloy isspecified by adding silicon, iron and/or nickel, a rare earth elementand zirconium so that solidification can be performed while maintaininga microstructure also when the rate of temperature rise is not extremelyhigh. Thus, high heat resistance and creep resistance can be implementedalso when the pressurized powder compact not yet worked into the productshape is exposed to a temperature of at least 450° C. for at least 15seconds and not more than 30 minutes.

[0029] While high heat resistance and creep resistance can beimplemented also when the time exposing the pressurized powder compactto a temperature of at least 450° C. is less than 15 seconds, theequipment cost is increased in this case.

[0030] In the aforementioned method of preparing a heat-resistant,creep-resistant aluminum alloy, the pressurized powder compact ispreferably solidified by hot plastic working at a rate of change(working rate) of at least 60% in average area of a sectionperpendicular to a pressurization axis for working the pressurizedpowder compact into the product shape.

[0031] Thus, a final product having a complicated shape can be readilymanufactured.

[0032] In the aforementioned method of preparing a heat-resistant,creep-resistant aluminum alloy, the hot plastic working preferablyincludes a step of performing solidification by hot forging.

[0033] Thus, a final product can be manufactured with high forgeability.

[0034] In the aforementioned method of preparing a heat-resistant,creep-resistant aluminum alloy, the step of working the pressurizedpowder compact into the product shape by the hot plastic workingpreferably includes steps of performing first heat treatment on thepressurized powder compact at a temperature of at least 420° C. and notmore than 550° C., performing powder forging on the pressurized powdercompact subjected to the first heat treatment thereby obtaining apowder-forged body, performing second heat treatment on thepowder-forged body at a temperature of at least 400° C. and not morethan 550° C., and working the powder-forged body subjected to the secondheat treatment into the product shape by shape forging.

[0035] Thus, an aluminum alloy excellent in heat resistance and heatcreep resistance can be obtained through two heating steps and twoforging steps.

[0036] In the aforementioned method of preparing a heat-resistant,creep-resistant aluminum alloy, the step of working the pressurizedpowder compact into the product shape by the hot plastic workingpreferably includes steps of performing heat treatment on thepressurized powder compact at a temperature of at least 450° C. and notmore than 550° C., performing powder forging on the pressurized powdercompact subjected to the heat treatment thereby obtaining apowder-forged body, and working the powder-forged body into the productshape by shape forging.

[0037] Thus, an aluminum alloy having microcrystal grains with excellentheat resistance and creep resistance can be obtained through a singleheating step and two forging steps.

[0038] In the aforementioned method of preparing a heat-resistant,creep-resistant aluminum alloy, the step of working the pressurizedpowder compact into the product shape by the hot plastic workingpreferably further includes steps of performing heat treatment on thepressurized powder compact at a temperature of at least 450° C. and notmore than 550° C., and working the pressurized powder compact subjectedto the heat treatment into the product shape by powder shape forging.

[0039] Thus, an aluminum alloy having microcrystal grains with excellentheat resistance and creep resistance can be obtained through a singleheating step and a single forging step.

[0040] In the aforementioned method of preparing a heat-resistant,creep-resistant aluminum alloy, the step of working the pressurizedpowder compact into the product shape by the hot plastic workingpreferably includes steps of performing first heat treatment on thepressurized powder compact at a temperature of at least 420° C. and notmore than 550° C., performing extrusion on the pressurized powdercompact subjected to the first heat treatment thereby obtaining anextruded body, cutting the extruded body, performing second heattreatment on the cut extruded body at a temperature of at least 400° C.and not more than 550° C., and working the extruded body subjected tothe second heat treatment into the product shape by shape forging.

[0041] Thus, an aluminum alloy having microcrystal grains with excellentheat resistance and creep resistance can be obtained by heating andextrusion.

[0042] A method of preparing a billet of a heat-resistant,creep-resistant aluminum alloy according to the present invention is amethod of preparing a billet of a heat-resistant, creep-resistantaluminum alloy containing at least 10 mass % and not more than 30 mass %of silicon, at least 3 mass % and not more than 10 mass % of at leasteither iron or nickel in total, at least 1 mass % and not more than 6mass % of at least one rare earth element in total and at least 1 mass %and not more than 3 mass % of zirconium while containing none oftitanium, magnesium and copper, with the rest substantially containingaluminum, comprising a step of molding rapidly cooled alloy powderconsisting of an aluminum alloy into a pressurized powder compact andthereafter performing hot plastic working on the pressurized powdercompact thereby forming a billet, while the time exposing thepressurized powder compact to a temperature of at least 450° C. beforeforming the billet is at least 10 seconds and within 20 minutes.

[0043] According to the inventive method of preparing a billet of aheat-resistant, creep-resistant aluminum alloy, an aluminum alloy havinga microcrystal grains with excellent heat resistance and creepresistance can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIGS. 1 to 3 are schematic perspective views showing first hotplastic working of a heat-resistant, creep-resistant aluminum alloyaccording to an embodiment of the present invention in order of steps.

[0045]FIGS. 4A, 4B and 5 are schematic perspective views showing secondhot plastic working of the heat-resistant, creep-resistant aluminumalloy according to the embodiment of the present invention in order ofsteps.

[0046]FIG. 6 illustrates a first method of preparing the heat-resistant,creep-resistant aluminum alloy according to the embodiment of thepresent invention.

[0047]FIG. 7 illustrates a second method of preparing theheat-resistant, creep-resistant aluminum alloy according to theembodiment of the present invention.

[0048]FIG. 8 illustrates a third method of preparing the heat-resistant,creep-resistant aluminum alloy according to the embodiment of thepresent invention.

[0049]FIG. 9 illustrates a fourth method of preparing theheat-resistant, creep-resistant aluminum alloy according to theembodiment of the present invention.

[0050]FIGS. 10, 11, 12A, 12B, 13A and 13B are perspective views forillustrating the shape of a billet for preparing the heat-resistant,creep-resistant aluminum alloy according to the embodiment of thepresent invention. FIG. 12B is a schematic sectional view taken alongthe line XII-XII in FIG. 12A, and FIG. 13B is a schematic sectional viewtaken along the line XIII-XIII in FIG. 13A.

[0051] FIGS. 14 to 18 illustrate heating patterns A to E respectively.

[0052]FIG. 19 illustrates creep deformation properties.

BEST MODES FOR CARRYING OUT THE INVENTION

[0053] An embodiment of the present invention is now described withreference to the drawings.

[0054] A heat-resistant, creep-resistant aluminum alloy according to thepresent invention contains at least 10 mass % and not more than 30 mass% of silicon, at least 3 mass % and not more than 10 mass % of at leasteither iron or nickel in total, at least 1 mass % and not more than 6mass % of at least one rare earth element (e.g., misch metal (MM)) intotal and at least 1 mass % and not more than 3 mass % of zirconium withthe rest consisting of aluminum and unavoidable impurities, andsubstantially contains no other additional elements. In the aluminumalloy, the mean crystal grain size of silicon is not more than 2 μm, themean grain size of compounds other than silicon is not more than 1 μm,and the mean crystal grain size of the aluminum matrix is at least 0.2μm and not more than 2 μm.

[0055] The aforementioned aluminum alloy, substantially containing noelements other than the aforementioned additional elements, may containother elements in a range not damaging heat resistance and creepresistance. For example, the aluminum alloy may contain at least 0.5mass % and not more than 5 mass % of at least one element selected froma group consisting of cobalt, chromium, manganese, molybdenum, tungstenand vanadium in total as other element(s). The aluminum alloy accordingto this embodiment contains none of titanium, magnesium and copperexerting bad influence on creep resistance and refinement of crystalgrains.

[0056] A preparation method according to this embodiment is nowdescribed.

[0057] The preparation method according to this embodiment is a methodof preparing a heat-resistant, creep-resistant aluminum alloy having theaforementioned composition.

[0058] In the method of preparing the heat-resistant, creep-resistantaluminum alloy having such a composition, rapidly cooled alloy powderconsisting of an aluminum alloy is first formed by atomization or thelike, for example. This rapidly cooled alloy powder is molded into apressurized powder compact, which in turn is worked into a product shapeby hot plastic working.

[0059] The steps of the hot plastic working are described with referenceto FIGS. 1 to 3.

[0060] Referring to FIG. 1, rapidly cooled alloy powder is molded toform a cylindrical pressurized powder compact 1 a, for example. Therelative density of this pressurized powder compact 1 a is about 80%,for example.

[0061] Referring to FIG. 2, this pressurized powder compact 1 a isheated and thereafter pressurized by hot forging (powder forging), forexample, thereby forming a dense forged body (billet) 1 b. The relativedensity of this dense forged body 1 b is 100%.

[0062] Referring to FIG. 3, this dense forged body 1 b is heated andthereafter pressurized by hot forging (shape forging), for example,thereby forming a pistonlike forged body (product) 1 c, for example,having the final product shape.

[0063] In the above description, powder forging is a step of removingmoisture adsorbed by the pressurized powder compact 1 a and increasingthe relative density to 100%, thereby obtaining the billet. In the abovedescription, further, shape forging is a step for working the billetinto the final product shape.

[0064] The time exposing the pressurized powder compact to a temperatureof at least 450° in the process for working the same into the finalproduct shape is at least 15 seconds and within 30 minutes.

[0065] Further, solidification is preferably performed by hot plasticworking (e.g., hot forging) with a working rate (rate of change of theaverage area of a section perpendicular to the pressurization axis) ofat least 60% for working the pressurized powder compact 1 a into theforged body 1 c having the final product shape.

[0066] The hot plastic working preferably includes a step of performingsolidification by a single or at least two steps of hot forging ashereinabove described.

[0067] Another exemplary hot plastic working including extrusion isdescribed with reference to FIGS. 4A, 4B and 5.

[0068] In this method, rapidly cooled alloy powder is first molded forforming a cylindrical pressurized powder compact 1 a, for example, asshown in FIG. 1. The relative density of this pressurized powder compact1 a is about 80%, for example.

[0069] Referring to FIGS. 4A and 4B, this pressurized powder compact 1 ais heated and thereafter worked by powder extrusion, for example,thereby forming an extruded body 1 b. The relative density of thisextruded body 1 b is 100%. This extruded body 1 b is cut.

[0070] Referring to FIG. 5, the extruded body 1 b is cut thereby forminga billet 1 b. This billet 1 b is heated and thereafter pressurized byhot forging (shape forging), for example, thereby forming a pistonlikeforged body (product) 1 c, for example, having the final product shapeshown in FIG. 3.

[0071] Thus, the billet may be formed not by powder forging but bypowder extrusion, to be thereafter worked into the final product shapeby shape forging.

[0072] These preparation methods are now described in detail as to fourpatterns.

[0073] Referring to FIG. 6, material powder consisting of rapidly cooledalloy powder having a prescribed composition is first prepared in thefirst preparation method. This material powder is subjected to powderpressurization molding (step S1), thereby forming the cylindricalpressurized powder compact 1 a shown in FIG. 1. The relative density ofthis pressurized powder compact 1 a is set to 80%. This pressurizedpowder compact 1 a is heated at a temperature of at least 420° C. andnot more than 550° C. At this time, the pressurized powder compact 1 ais heated at a temperature of at least 460° C. and not more than 500° C.for at least 15 seconds and within 15 minutes, under more preferableconditions (step S2). The heated pressurized powder compact 1 a issubjected to hot forging (powder forging) (step S3). In this powderforging, the pressurized powder compact 1 a is so worked that therelative density reaches 100% and the area of a section of thepressurized powder compact 1 a perpendicular to a compression axisremains unchanged. Thus, the dense forged body (billet) 1 b shown inFIG. 2 is obtained. This billet 1 b is heated at a temperature of atleast 400° C. and not more than 550° C. At this time, the billet 1 b isheated at a temperature of at least 400° C. and not more than 500° C.for at least 15 seconds and within 15 minutes under more preferableconditions (step S4). The heated billet 1 b is subjected to hot forging(shape forging) (step S5). In this shape forging, the billet 1 b isworked into the final product shape so that the area of the section ofthe billet 1 b perpendicular to the compression axis changes within therange of at least 60% and not more than 90%. Thus, the pistonlike forgedbody (product) 1 c, for example, having the final product shape shown inFIG. 3 is formed.

[0074] Referring to FIG. 7, material powder consisting of rapidly cooledalloy powder having a prescribed composition is first prepared in thesecond preparation method. This material powder is subjected to powderpressurization molding (step S1), thereby forming the cylindricalpressurized powder compact 1 a shown in FIG. 1. The relative density ofthis pressurized powder compact 1 a is set to 80%. This pressurizedpowder compact 1 a is heated at a temperature of at least 450° C. andnot more than 550° C. At this time, the pressurized powder compact 1 ais heated at a temperature of at least 460° C. and not more than 520° C.for at least 15 seconds and within 30 minutes, under more preferableconditions (step S2). The heated pressurized powder compact 1 a issubjected to hot forging (powder forging) (step S3). In this powderforging, the pressurized powder compact 1 a is so worked that therelative density reaches 100% and the area of a section of thepressurized powder compact 1 a perpendicular to a compression axisremains unchanged. Thus, the dense forged body (billet) 1 b shown inFIG. 2 is obtained. This billet 1 b is subjected to hot forging (shapeforging) (step S5). In this shape forging, the billet 1 b is worked intothe final product shape so that the area of the section of the billet 1b perpendicular to the compression axis changes within the range of atleast 60% and not more than 90%. Thus, the pistonlike forged body(product) 1 c, for example, having the final product shape shown in FIG.3 is formed.

[0075] Referring to FIG. 8, material powder consisting of rapidly cooledalloy powder having a prescribed composition is first prepared in thethird preparation method. This material powder is subjected to powderpressurization molding (step S1), thereby forming the cylindricalpressurized powder compact 1 a shown in FIG. 1. The relative density ofthis pressurized powder compact 1 a is set to 80%. This pressurizedpowder compact 1 a is heated at a temperature of at least 450° C. andnot more than 550° C. At this time, the pressurized powder compact 1 ais heated at a temperature of at least 460° C. and not more than 520° C.for at least 15 seconds and within 30 minutes, under more preferableconditions (step S2). The heated pressurized powder compact 1 a issubjected to hot forging (powder shape forging) (step S3 a). In thispowder shape forging, the pressurized powder compact 1 a is so workedinto the final product shape that the relative density reaches 100% andthe area of a section of the billet 1 b perpendicular to a compressionaxis changes within the range of at least 60% and not more than 90%.Thus, the pistonlike forged body (product) 1 c, for example, having thefinal product shape shown in FIG. 3 is formed.

[0076] Referring to FIG. 9, material powder consisting of rapidly cooledalloy powder having a prescribed composition is first prepared in thefourth preparation method. This material powder is subjected to powderpressurization molding (step S1), thereby forming the cylindricalpressurized powder compact 1 a shown in FIG. 1. The relative density ofthis pressurized powder compact 1 a is set to 80%. This pressurizedpowder compact 1 a is heated at a temperature of at least 420° C. andnot more than 550° C. At this time, the pressurized powder compact 1 ais heated at a temperature of at least 450° C. and not more than 500° C.for at least 15 seconds and within 15 minutes, under more preferableconditions (step S2). The heated pressurized powder compact 1 a issubjected to extrusion as shown in FIGS. 4A and 4B (step S11). In thisextrusion, the pressurized powder compact 1 a is so worked that therelative density reaches 100% and the area of a section of thepressurized powder compact 1 a perpendicular to a compression axischanges within the range of at least 75% and not more than 90%.Thereafter the extruded body 1 b is cut (step S12), thereby obtainingthe billet 1 b shown in FIG. 5. This billet 1 b is heated at atemperature of at least 400° C. and not more than 550° C. At this time,the billet 1 b is heated at a temperature of at least 400° C. and notmore than 500° C. for at least 15 seconds and within 15 minutes, undermore preferable conditions (step S4). The heated billet 1 b is subjectedto hot forging (shape forging) (step S5). In this shape forging, thebillet 1 b is worked into the final product shape so that the area ofthe section of the billet 1 b perpendicular to the compression axischanges within the range of at least 60% and not more than 90%. Thus,the pistonlike forged body product) 1 c, for example, having the finalproduct shape shown in FIG. 3 is formed.

[0077] The billet obtained according to this embodiment is nowdescribed.

[0078] In any of the aforementioned first to fourth preparation methods,the cylindrical billet 1 b shown in FIG. 2 or FIG. 5 is obtained. Thecylindrical shape includes not only a discoidal shape having a smallthickness (length) T with respect to the diameter D as shown in FIG. 10but also a columnar shape having a large thickness (length) T withrespect to the diameter D as shown in FIG. 11. It is assumed that thecylindrical shape in the present invention also includes shapes, notcompletely cylindrical, having small dents on the front and rearsurfaces as shown in FIGS. 12A and 12B and having small projections onthe front and rear surfaces as shown in FIGS. 13A and 13B, for example.

[0079] The billet of a heat-resistant, creep-resistant aluminum alloyaccording to this embodiment has the composition containing at least 10mass % and not more than 30 mass % of silicon, at least 3 mass % and notmore than 10 mass % of either iron or nickel in total, at least 1 mass %and not more than 6 mass % of at least one rare earth element (e.g.,misch metal (MM)) in total and at least 1 mass % and not more than 3mass % of zirconium while containing none of titanium, magnesium andcopper, with the rest consisting of aluminum and unavoidable impurities.

[0080] This billet 1 b may contain other elements in a range notdamaging heat resistance and creep resistance. For example, the billetmay contain at least 0.5 mass % and not more than 5 mass % of at leastone element selected from a group consisting of cobalt, chromium,manganese, molybdenum, tungsten and vanadium in total as otherelement(s).

[0081] The powder-forged billet 1 b prepared according to the first orsecond preparation method has tensile strength of at least 230 MPa andnot more than 260 MPa at 300° C., elongation of at least 1% and not morethan 7% at 300° C., and hardness of at least 77 and not more than 92 inHRB (B scale of Rockwell hardness) at the room temperature. The grainsize of Si in the structure of this powder-forged billet 1 b is at least1.0 μm and not more than 1.6 μm, the grain sizes of compounds other thanSi are at least 0.5 μm and not more than 0.7 μm, and the grain size ofAl is at least 0.3 μm and not more than 0.5 μm.

[0082] The extruded/cut billet 1 b prepared according to the fourthpreparation method has tensile strength of at least 220 MPa and not morethan 250 MPa at 300° C., elongation of at least 7% and not more than 15%at 300° C., and hardness of at least 74 and not more than 88 in HRB atthe room temperature. The grain size of Si in the structure of thisextruded/cut billet 1 b is at least 1.1 μm and not more than 1.7 μm, thegrain sizes of compounds other than Si are at least 0.6 μm and not morethan 0.8 μm, and the grain size of Al is at least 0.4 μm and not morethan 0.6 μm.

[0083] The product 1 c having the final shape shown in FIG. 3 hastensile strength of at least 215-MPa and not more than 247 MPa 300° C.,elongation of at least 9% and not more than 14% at 300° C., and hardnessof at least HRB 72 and not more than HRB 88 at the room temperature. Thegrain size of Si in the structure of this product 1 c having the finalshape is at least 1.1 μm and not more than 1.7 μm, the grain sizes ofcompounds other than Si are at least 0.6 μm and not more than 0.8 μm,and the grain size of Al is at least 0.4 μm and not more than 0.6 μm.

[0084] Experimental Example of the present invention is now described.

[0085] Rapidly cooled alloy powder materials having compositions ofsamples Nos. 1 to 44 shown in Table 1 were prepared by air atomizationand molded to prepare pressurized powder compacts of φ80×21 mm.Pistonlike forged bodies having final shapes were prepared from thepressurized powder compacts by combinations of the following heatingpatterns A to E and hot plastic working a to e.

[0086] Referring to Table 1, misch metal (MM) was composed of 25 mass %of lanthanum (La), 50 mass % of cerium (Ce), 5 mass % of praseodymium(Pr) and 20 mass % of neodymium (Nd). TABLE 1 Sam- Hot pleComposition(Mass %) Heating Plastic No. Si Fe Ni Zr MM Cu Mg Cr Mn Mo CoW V Pattern Working Inventive 1 11 5 3 1.2 5 A a Sample 2 11 2 4 2.5 4 Aa 3 14 5 2 1.2 5 A a 4 14 2 3 2 4 A a 5 17 4 1.5 5 A a 6 17 3 0.5 1.5 5A a 7 17 2 1.5 1.5 5.5 A a 8 17 1 2 1.2 5.5 A a 9 17 3 1.5 5 A a 10 20 41.5 4 A a 11 20 3 0.5 1.5 4 A a 12 20 2 1.5 1.2 5 A a 13 20 1 2 1.2 5.5A a 14 20 3 1.2 5 A a 15 25 3 0.5 1.5 2 A a 16 25 2 1.5 1.2 5 A a 17 251 2 1.2 5 A a 18 25 3 1.2 3 A a 19 17 2 1.5 1.5 5 0.1 0.3 A a 20 17 21.5 1.5 5 0.5 0.3 A a 21 20 2 1.5 1.2 5 0.8 A a 22 20 2 1.5 1.2 5 0.20.6 A a 23 20 2 1.5 1.2 5 B a 24 20 2 1.5 1.2 5 C a 25 17 2 1.5 1.5 5 Ab 26 17 2 1.5 1.5 5 A c 27 17 2 1.5 1.5 5 A d 28 17 2 1.5 1.5 5 A e 2920 2 1.5 1.2 5 D a Comparative 30 20 2 1.5 1.2 5 E a Sample 31 17 2 1.51.5 5 1 A a 32 17 2 1.5 1.5 5 0.8 A a 33 17 1 2 1.2 5 0.5 0.06 A a 34 171 2 1.2 5 0.1 A a 35 8 8 1.5 5 A a 36 32 4 2 1.2 3 A a 37 11 12 1.2 5 Aa 38 20 0.5 0.5 1.5 5 A a 39 20 3 2 0 5 A a 40 17 2 1.5 1.5 0.7 A a 4117 2 0 0 2 4 0.5 A a 42 17 2 0 0 8 4 0.5 A a 43 12 5 3 2 A a 44 17 5 1 3A a

[0087] The aforementioned heating patterns A to E were set as follows:

[0088] The times for heating the samples from 450° C. to 500° C. wereset to 600 seconds in the heating pattern A as show in FIG. 14, to 1500seconds in the heating pattern B as shown in FIG. 15, to 25 seconds inthe heating pattern C as shown in FIG. 16, to 5 seconds in the heatingpattern D as shown in FIG. 17, and to 2000 seconds in the heatingpattern E as shown in FIG. 18.

[0089] The rates for heating the samples from 20° C. to 450° C. in therespective heating patterns A to E were set identical to the rates forheating the samples from 450° C. to 500° C. in the respective heatingpatterns.

[0090] In the hot plastic working a, the pressurized powder compact 1 aof φ80×21 mm shown in FIG. 1 was worked into the dense forged body 1 bof φ80×16 mm shown in FIG. 2 by hot forging, and this dense forged body1 b was further worked into the pistonlike forged body 1 c of φ80 mmshown in FIG. 3 by hot forging. The working rate in this pistonlikeforged body 1 c was set to 67%.

[0091] In the hot plastic working b, the pressurized powder compact 1 aof φ80×21 mm shown in FIG. 1 was worked into the pistonlike forged body1 c of φ80 mm shown in FIG. 3 by hot forging. The working rate in thispistonlike forged body 1 c was set to 67%.

[0092] In the hot plastic working c, the pressurized powder compact 1 aof φ80×21 mm shown in FIG. 1 was worked into the dense forged body 1 bof φ80×16 mm shown in FIG. 2 by hot forging, and this dense forged body1 b was further worked into the pistonlike forged body 1 c of φ80 mmshown in FIG. 3 by hot forging. The working rate in this pistonlikeforged body 1 c was set to 75%.

[0093] In the hot plastic working d, the pressurized powder compact 1 aof φ80×21 mm shown in FIG. 1 was worked into the dense forged body 1 bof φ80×16 mm shown in FIG. 2 by hot forging, and this dense forged body1 b was further worked into the pistonlike forged body 1 c of φ80 mmshown in FIG. 3 by hot forging. The working rate in this pistonlikeforged body 1 c was set to 50%.

[0094] In the hot plastic working e, the pressurized powder compact 1 aof φ80×21 mm shown in FIG. 1 was worked into the pistonlike forged body1 c of φ80 mm shown in FIG. 3 by hot forging. The working rate in thispistonlike forged body 1 c was set to 50%.

[0095] As to the forged bodies having the final shapes obtained in theaforementioned manner, tensile strength values at 300° C., elongationvalues at 300° C. and minimum creep rates following application oftension of 80 MPa at 300° C. were measured. As to the forged bodieshaving the final shapes obtained in the aforementioned manner, further,mean crystal grain sizes of silicon, mean grain sizes of compounds otherthan silicon and mean crystal grain sizes of aluminum matrices weremeasured. Tables 2 and 3 show the results. TABLE 2 Evaluated Items 300°C. 300° C. 80 MPa Si Grain Size of Al Tensile 300° C. Minimum GrainCompound Grain Sample Strength Elongation Creep Rate Size Other thanSize No. (MPa) (%) (l/s) (μm) Si (μm) (μm) Inventive 1 220 12.2 7.70 ×10⁻⁹ 1.2 0.8 0.6 Sample 2 215 13.5 8.50 × 10⁻⁹ 1.1 0.8 0.6 3 227 12.66.00 × 10⁻⁹ 1.3 0.8 0.6 4 225 12 5.60 × 10⁻⁹ 1.3 0.8 0.6 5 216 11.4 3.80× 10⁻⁹ 1.4 0.7 0.6 6 228 12.2 4.20 × 10⁻⁹ 1.3 0.8 0.5 7 224 11.6 4.00 ×10⁻⁹ 1.5 0.7 0.6 8 220 12 4.40 × 10⁻⁹ 1.5 0.7 0.5 9 232 10.8 3.70 × 10⁻⁹1.5 0.8 0.6 10 235 10 3.30 × 10⁻⁹ 1.6 0.7 0.5 11 224 12 3.40 × 10⁻⁹ 1.50.7 0.5 12 242 10.2 3.20 × 10⁻⁹ 1.6 0.7 0.5 13 230 11 3.60 × 10⁻⁹ 1.60.6 0.5 14 233 11 3.10 × 10⁻⁹ 1.4 0.7 0.4 15 245 9.8 2.90 × 10⁻⁹ 1.6 0.70.5 16 240 10.4 2.70 × 10⁻⁹ 1.7 0.7 0.4 17 247 9.6 2.80 × 10⁻⁹ 1.7 0.60.5 18 244 10 2.60 × 10⁻⁹ 1.6 0.6 0.5 19 235 11 3.50 × 10⁻⁹ 1.6 0.7 0.520 233 10.7 3.30 × 10⁻⁹ 1.6 0.7 0.5 21 236 10.4 2.90 × 10⁻⁹ 1.5 0.7 0.622 239 10 2.80 × 10⁻⁹ 1.5 0.8 0.6 23 230 11 3.60 × 10⁻⁹ 1.4 0.8 0.5 24222 12.4 3.80 × 10⁻⁹ 1.6 0.7 0.5 25 227 12 4.20 × 10⁻⁹ 1.5 0.8 0.5 26228 11.3 4.50 × 10⁻⁹ 1.4 0.7 0.6 27 215 13 4.40 × 10⁻⁹ 1.4 0.8 0.6 28216 13.1 4.80 × 10⁻⁹ 1.6 0.7 0.6 29 240 9.9 3.20 × 10⁻⁹ 1.2 0.8 0.4

[0096] TABLE 3 Evaluated Item 300° C. 300° C. 80 MPa Si Grain Size of AlTensile 300° C. Minimum Grain Compound Grain Sample Strength ElongationCreep Rate Size Other than Size No. (MPa) (%) (l/s) (μm) Si (μm) (μm)Comparative 30 175 18 8.80 × 10⁻⁸ 2.7 1.4 2.2 Sample 31 220 11 9.20 ×10⁻⁸ 1.5 0.8 0.5 32 225 12.2 9.50 × 10⁻⁸ 1.6 0.8 0.5 33 214 14 1.20 ×10⁻⁷ 1.5 0.7 0.6 34 220 12.3 5.00 × 10⁻⁸ 1.5 0.7 0.5 35 207 13 4.00 ×10⁻⁸ 1.4 1.3 1.9 36 235 5 4.40 × 10⁻⁸ 2.3 1.3 1.8 37 233 3.9 5.00 × 10⁻⁸1.6 1.8 2.5 38 230 5.3 1.10 × 10⁻⁷ 3.3 1.5 2.3 39 235 8.5 5.80 × 10⁻⁸1.4 1.5 2.2 40 209 11.1 8.50 × 10⁻⁸ 2.2 0.9 1.4 41 225 11.1 8.30 × 10⁻⁸1.5 0.8 1.1 42 233 9.9 7.00 × 10⁻⁸ 1.6 0.8 1.1 43 208 9.9 6.80 × 10⁻⁸ 21 1.4 44 192 5.3 7.20 × 10⁻⁸ 2.2 0.9 1.3

[0097] Referring to Tables 2 and 3, the term “minimum creep rate”indicates the minimum inclination in a creep deformation property curvefollowing measurement of strain varying with time under a constanttemperature and a constant load, as shown in FIG. 9.

[0098] From the results shown in Tables 2 and 3, it has been proved thateach of the inventive samples Nos. 1 to 29 has high tensile strength ofat least 215 MPa at 300° C., large elongation of at least 9.6% at 3000and a low minimum creep rate of not more than 8.50×10⁻⁹ followingapplication of tension of 80 MPa at 300° C. It has been also proved thatthe mean crystal grain size of silicon is not more than 2 μm, the meangrain size of compounds other than silicon is not more than 1 μm and themean crystal grain size of the aluminum matrix is at least 0.2 μm andnot more than 2 μm in each of the inventive samples Nos. 1 to 29.

[0099] In each of comparative samples Nos. 30 to 44, the minimum creeprate was in excess of 8.50×10⁻⁹ following application of tension of 80MPa at 300° C. Tensile strength at 300° C. was lower than 215 MPa as toeach of comparative samples Nos. 30, 33, 35, 40, 43 and 44, whileelongation at 300° C. was smaller than 9.6% in each of comparativesamples Nos. 36 to 39 and 44.

[0100] From the above results, it has been proved that an aluminum alloyhaving a composition in the range of the present invention attainsexcellent characteristics as to all of tensile strength at 300° C.,elongation at 300° C. and the minimum creep rate following applicationof tension of 80 MPa at 300° C.

[0101] According to the heat-resistant, creep-resistant aluminum alloyand the method of preparing the same according to the present invention,as hereinabove described, excellent heat resistance and creep resistancecan be attained due to the prescribed composition and the prescribedstructure, whereby an aluminum alloy suitable as a piston or an enginepart employable at a high temperature (particularly in excess of 300°C.) and required to have high creep resistance and a method of preparingthe same can be obtained.

[0102] The embodiment and Experimental Example disclosed this time mustbe considered illustrative and not restrictive in all points. The scopeof the present invention is shown not by the above description but bythe scope of claim for patent, and it is intended that all modificationsin meanings and ranges equivalent to the scope of claim for patent areincluded.

Industrial Availability

[0103] As hereinabove described, the present invention is suitablyapplied to a member such as a piston, for example, required to have heatresistance and creep resistance.

What is claimed is:
 1. A method of preparing a heat-resistant,creep-resistant aluminum alloy containing at least 10 mass % and notmore than 30 mass % of silicon, at least 3 mass % and not more than 10mass % of at least either iron or nickel in total, at least 1 mass % andnot more than 6 mass % of at least one rare earth element in total andat least 1 mass % and not more than 3 mass % of zirconium with the restsubstantially consisting of aluminum, comprising a step of moldingrapidly cooled alloy powder consisting of an aluminum alloy into apressurized powder compact (1 a) and thereafter working said pressurizedpowder compact (1 a) into a product shape (1c) by hot plastic working,wherein the time exposing said pressurized powder compact (1 a) not yetworked into said product shape (1c) to a temperature of at least 450° C.is at least 15 seconds and within 30 minutes.
 2. The method of preparinga heat-resistant, creep-resistant aluminum alloy according to claim 1,performing solidification by hot plastic working at a rate of change ofat least 60% in average area of a section perpendicular to apressurization axis for working said pressurized powder compact (1 a)into said product shape (1c).
 3. The method of preparing aheat-resistant, creep-resistant aluminum alloy according to claim 1,wherein said hot plastic working includes a step of performingsolidification by hot forging.
 4. The method of preparing aheat-resistant, creep-resistant aluminum alloy according to claim 1,wherein said step of working said pressurized powder compact (1 a) intosaid product shape (1 c) by said hot plastic working includes steps of:performing first heat treatment on said pressurized powder compact (1 a)at a temperature of at least 420° C. and not more than 550° C.,performing powder forging on said pressurized powder compact (1 a)subjected to said first heat treatment thereby obtaining a powder-forgedbody (1 b), performing second heat treatment on said powder-forged body(1 b) at a temperature of at least 400° C. and not more than 550° C.,and working said powder-forged body (1 b) subjected to said second heattreatment into said product shape (1c) by shape forging.
 5. The methodof preparing a heat-resistant, creep-resistant aluminum alloy accordingto claim 1, wherein said step of working said pressurized powder compact(1 a) into said product shape (1 c) by said hot plastic working includessteps of: performing heat treatment on said pressurized powder compact(1 a) at a temperature of at least 450° C. and not more than 550° C.,performing powder forging on said pressurized powder compact (1 a)subjected to said heat treatment thereby obtaining a powder-forged body(1 b), and working said powder-forged body (1 b) into said product shape(1 c) by shape forging.
 6. The method of preparing a heat-resistant,creep-resistant aluminum alloy according to claim 1, wherein said stepof working said pressurized powder compact (1 a) into said product shape(1 c) by said hot plastic working further includes steps of: performingheat treatment on said pressurized powder compact (1 a) at a temperatureof at least 450° C. and not more than 550° C., and working saidpressurized powder compact (1 a) subjected said to heat treatment intosaid product shape (1 c) by powder shape forging.
 7. The method ofpreparing a heat-resistant, creep-resistant aluminum alloy according toclaim 1, wherein said step of working said pressurized powder compact (1a) into said product shape (1 c) by said hot plastic working includessteps of: performing first heat treatment on said pressurized powdercompact (1 a) at a temperature of at least 420° C. and not more than550° C., performing extrusion on said pressurized powder compact (1 a)subjected to said first heat treatment thereby obtaining an extrudedbody (1 b), cutting said extruded body (1 b), performing second heattreatment on cut said extruded body (1 b) at a temperature of at least400° C. and not more than 550° C., and working said extruded body (1 b)subjected to said second heat treatment into said product shape (1 a) byshape forging.
 8. A method of preparing a billet (1 b) of aheat-resistant, creep-resistant aluminum alloy containing at least 10mass % and not more than 30 mass % of silicon, at least 3 mass % and notmore than 10 mass % of at least either iron or nickel in total, at least1 mass % and not more than 6 mass % of at least one rare earth elementin total and at least 1 mass % and not more than 3 mass % of zirconiumwhile containing none of titanium, magnesium and copper, with the restsubstantially containing aluminum, comprising a step of molding rapidlycooled alloy powder consisting of an aluminum alloy into a pressurizedpowder compact (1 a) and thereafter performing hot plastic working onsaid pressurized powder compact (1 a) thereby forming a billet (1 b),wherein the time exposing said pressurized powder compact (1 a) to atemperature of at least 450° C. before forming said billet (1 b) is atleast 10 seconds and within 20 minutes.